Fading compensation radio signaling system



SPL 9, 1952 D. s. BOND ETAL 2,610,292

FADING COMPENSATION RADIO SIGNALING SYSTEM 3 Sheets-Sheet 1 Filed Marchl2, 1946 Tlc.

/mao/ A @404/ ATTORNEY Sept. 9, 1952 Filed March 12, 1946 D. S. BOND ETAL FADING COMPENSATION RADIO SIGNALING SYSTEM 5 Sheets-Sheet 2 Sept. 9,1952 D. s. BOND ET AL 2,610,292

FADING COMPENSATION RADIO' SIGNALING SYSTEM Tltl.

Patented Slept.. 9, 1952 Y f vFADnSTc. coMPNsSA'r YSTEM Donald S. Bond,Philadelphia, Pa., and Leland E.

Thompson, Merchantville, N. Radio Corporation of America,

Delaware J., assignors to a corporation of Application March 12, 1946,Serial No. 653,726 o v (c1. 25o-6) f L 7 Claims.

1 The'present inventionrelates to radio signalling systems and `moreparticularly to those adapted for use in the ultra short Wave region.

An'object ofthe present invention is to reducefading of microwaves.Another object of thepresent invention is the reduction of interferencein ultra short Wave frequency signalling system due to multipathpropagation.

A further object of the present invention is the improvement of radiorelaying communication systems.

Still afurther object of the present invention is the provision of adiversity system, particularly adapted for use with relay systems suchas described in the copending application of L. E.

Thompson, Serial Number 576,453, filed February 6, 1945, issued July l1,1950,v as U. S. Patent at a receiving point, one above the other, theheights being so chosen that the lower or normal ray is received muchbetter by the upper antenna. Then when multipath phenomena causedestructive interfence at the upper antenna the abnormal ray produces amuch stronger signal at the lower antenna than in the unperturbedcondition. Alternatively, the heights of the rtwo antennas, one abovethe other, maybe so chosen that each antenna receives a signal ofsubstantially the same strength under normal conditions. Under multipathconditions when, for example,l two wavesjarrive at one of the antennasexactly 1.80 degrees out of phase with each other, the arrival of thetwo waves at the other antenna will not be 180 degrees out of phase butwill be agreater or less amount so that there will be a usefulresultant. The outputs from the two antennas are combined Iby adiversity combining means to obtain a signal substantially free fromthis type of fading. Y

The present invention is particularly adapted for but not limited to usein relay systems wherein a double angle modulated wave is used. Byanglemodulation we mean that type of modulation wherein a characteristicof a continuous wave other than its amplitude is varied in accordancewith a signal. 'More specically, the -anglemodulation may be purefrequency modul lation Vor pure phase'modulation of some type vofmodulation having both components. By double angle modulation ismeant asystem in which oneor more signalchannels are used to angle,

frequency or phase modulate a common sub'- carrier frequency and thiscommon modulated frequency is then employed to angle, vfrequency orphase modulate a wave of still higher frequencyfof a value suitable forradio transmission.

-Single angle modulation or amplitude modulation of the transmittedcarrier may be used if desired. Furthermore, theuse of our presentinvention for relaying or broadcast reception of television signals isspecifically contemplated.

In `the followingY description and in the accom#- panying drawingscertain values of frequency may be givenV for the carrier channels,'heights of towers, relay distances, etc.V However, it is to be clearlyunderstood that these valueshave been chosen so as'to present a typicalexample which may be varied but obviously widely different choices as tofrequency and other values may be made. Hence, the invention is notto beVconsidered as limited to the values chosen for'illustrative purposes. I

The following detailed vdescription is accom- 'panied-by a Vdrawing inwhich:

Figures 1 and 2 illustrate in plan View a. section ofr the earthssurface with transmitting and receiving locations indicated thereon. Thecurvature of the earth is enormously exaggerated in order to facilitateexplanation of the invention; 'f Figure 3 illustrates in block `diagramforman embodiment of the present invention;v

' Figure 4 illustrates a modification thereof;-

` Figure 5 illustrates in a chart formthe combining actionin the outputof the system shown in Figure 3 ywhereby a substantially constant signalamplitude is obtained. and

Figure 6 illustrates a further modification of the present invention,while '7 A Figures '7 and 8 illustrate in block diagram form two furtherdiversity systems which vmay be used with one vertically spaced antennaarrangement to overcome fading, while Figure 9 illustrates in plan viewan arrangement of transmitting .and receiving' antennas useful inunderstanding the'present invention,

and

.o Figure 10 illustrates a further modification of the presentinvention.

Reference will be made to Figure 1 wherein E denotes the surface of theearth. Located thereon is a transmitting station having a transmittingantenna TA mountedl at the top of a suitable tower T. The antennay ispreferably highlydirective and may include a parabolic reflector orother wave directing structure .j A.

be the input of a relay station in which caseY further transmittingantennas directed toward further receiving equipment'may be used. Now-11t is known that in the lower troposphere surrounding the earth theindex of refraction may vary with height changes from time to time.

between the transmitting antenna and receiving antenna in Figure 1follows a curved path A having the radius R1 somewhat greater thanthatof the earth. It has been found that under -unperturbed conditions ofthe lower atmosphere Vthewalue of radius R1 is approximately four times;that ofR, the earths radius. Now, if a gradient exists .of a typecausing the ray path to, bendmore at greater elevation abovethe earthssurface. a second pathB of radius R2 may exist.l ARadius R2 is, lessthan radius R1. Therefore, the two rays travelling over paths A vandiVarrive at the receiving antenna RA in differing phase relationships. Ifthis difference is an Jodd multiple of Aa half wavelength, destructiveinterference occurs causing a minimum value of signalwhich may be verynear to zero because `of thesubstantialequality of attenuation along thepaths A andB.

Computations based on the geometry of Fgure lfdisclose that path B needlie only a few Vs or 100s of feet above path A as indicated by thedistance d for path lengths of to 40 Vmiles in order to produce thefirst minimum in which the path difference is one half a wavelength, Asatmospheric conditions change such that radius R2 becomes less and less,the distance between TA and RA for which destructive interference nrs'toccurs becomes substantially less. For example, at a distance of 30miles where R1;.4R, then when R2==R, the rst minimum ,occurs f forafrequency of 3,300 megacycles per second.

Experimental results have verified this picture of variations of signalintensity. There is a type ofcfading that is observed under manyconditions, especially in summer months, that is characterized by arapid diminution in signal from its norfrna'lfr unperturbed value tosubstantially zero intensity. There may be variations during'which thesignal rises for short durations of time above the normal value and atsomewhat later time the signal may reduce rapidly to a normal value.Such a'period of fading mayoccupy a few minlutes' Vr perhaps up to Vhalfan hour under the typical propagation conditions encountered. The natureof thefading phenomena suggests the cancellation of signals along two ormore paths, the signals being individually rather strong. c

From the geometry of ray paths A and B vof Figure 1 it may be determinedthat the height of antenna TA or antenna RA determines whether the twosignals are in phase or out at any Yparticular instant of time.According to 'an aspect of the present invention it is proposed toinstall one or more additional antennas at either the -receiving ortransmitting location or lboth, in order to provide two or more circuitswhose output may be combined in a diversity fashion...)

In Figure 2 is shown an arrangement wherein spaced antennas are used atthe receiving location only. However, it will be understood that thesame results may be achieved if the separation is effected at thetransmitter location instead,or at both loctions.` Thus, in Figure 2 apair of receiving antenna -RA1 land RAz are mounted on the receivingtower T. The second Vantenna RAz is installed at a lower elevation onthereceiving tower. The location is so chosen in this illustration thatunder conditions in which the medium is unperturbed, the path Arepresents f a ray whose intensityat RA2 is rather small. This .resultwill be obtained when the receiving antenna RAz is located below theeffective horizon for path A. 'It is so shown in the figure, path Aintersecting the surface of earth E before arriving at theV receivinglocation. 'I'hen when abnormal propagation of the type resulting in theundesired phenomenon, as described above, is encountered, ray B will bereceived at antenna RAz `with an intensity -comparable to that obtainedat RA1.. However, the interference phenomena will not be the same asbefore because under this condition only Aray B will be received at thereceiving antenna RAZ. Thus, antenna RA1 may be receiving waves over twopaths and arriving degrees out of phase so as to result in a zero signalwhile antenna RAz will be receiving a strong signal represented by ray4B in Figure 2. If the outputs of the antenna systems RA1 and RAz arecombined as described hereafter the resultant output is substantiallyless affected by the interference'type of fading described.

Experiments We have conducted have demonstrated that the signal of thereceiving antenna RA1 need not be zero in order to receive the benefitsof the antenna RAzl It appears sufficientto establish an appreciabledifference in heights of the antennas to secure conditions in whichfading is not simultaneous.. For example, over a 28 mile path, for whichthe transmitting antenna is located approximately 2'5'0 .feet -above thesurface of the earth, it is suicient to locate the receiving antenna RA1at a height of 100 feet abovethe surface of the earth and receivingantenna RAz ata height of 50 feet in order to secure marked diminutionof the fading phenomena.

One method of combining the outputs of antennas RA1 and YRAzwill now bedescribed with reference to Figure 3. The system shown in this figure isespecially suited to the system of radio signalling employing a doublefrequency-modulation scheme with a sub-*carrier as described in thecopending application of L. E. Thompson, Serial Number 576,453, filed`February 6, 1945. Receiving antennasRA-300 vand RA-30I located asindicated in Figure 2 arearranged to receive signals from thetransmitting antenna TA. The communication wave energy Y from antennaRA-300 is heterodyned in converter 302 with a wave from a local Vbeatingoscillator 304 to produce intermediate frequency which may be of theorder of 30 megacycles plus or minus one megacycle. The waves ofintermediate frequency are amplified in the intermediate amplifier 306and then fed to a discriminator 308. Similarly the wave energy receivedat receiving .antenna RA-30| is heterodyned in converter 302 with a wavefrom a local Ybeating-oscillator 304 in order to produce a secondvintermediate frequency of 30 megacycles per second. If desiredoscillator 304 maybe used lfor .both converters 302 and 302. The ,wavesof. intermediate frequency from the converter 302' are amplified in theintermediate frequency `amplifier 306,y and fed to discriminator 308'.The output of discriminator 308 and 308 consists of sub-carrier signalsof identical characteristics each frequency modulated, so as to cover aband of plus or minus 400 kilocycles per second about a base frequencyof 1.0 megacycle per second. Due to the existence of limiters in each ofthe intermediate amplifiers 306 and 300', the level of the sub-carrierin each channel is substantially constant until the rsignal Vthresholdof the radioV frequency system is reached. A further decrease of thesignal. however, then causes an'abruptdiminutionvof the sub-carrier.Then, when the two sub-carriers are y combined in a single channel309'they may be additive if they are substantially-of the Vsaine levelor only the stronger may be significant in the following stages. If one.channel loses its input altogether the noise will not rise to the pointwhere it rinterferes with the sub-carrier of 4the other channel by thenature of the frequenc modulationgsignal characteristics. y e Y l Thisis illustrated in Figure 5 wherein the signal level of the sub-carriersthrough the two channels 308 and 303' are indicated by curves 400 and40|. The maximum noise level in the channels is indicated by the crosshatched area below line 403. It will be noted from these curves .thatsmall diierences in amplitude of the subcarriers represented by curves400 and 40| are of no significance because of the limiting action Aofthe sub-carrier that occurs in the nal demodulation process. Thus, withno auxiliary combining circuits, it becomes possible to obtain inaccordance with the methods described above.

In the receiving location, it may be desirable to place all of theantenna arrangements; that is, the antennas RA-300 and RA-30I, closetogether. However, dueto the spacing between the transmitting antennasTA1 and TAz the diversity effect will be still realized. As a furthermodification, -the receiving antennas 300 and 30| may also be separated.The receiving structure operates just as described above with referenceto Figure 3.

Alternatively, if both antennas RA1 and RAe are so located as to providea normal line of sight -path to the transmitter, normal propagationprovides about the same received signal at each antenna. However, whenabnormal conditions ensue, the phase of the received signals on each ofc the antennas RA1 and RAZ is not such as to cancel at both antennas atthe same time. This may be explained by reference to Figure 9 where forconvenience the ray paths are shown as straight rlines and anlequivalent reflecting plane C is shown instead of the curved paths A andB of different radius of curvature as shown in Figure 1. Asignal overpaths A and B arrives at antenna RA1 and a signal over paths A1 and B1arrives at antenna RAz. If the reecting plane LC is at a particularheight above the earth, the

path length B will be a half wave-length longer than the path A. Thenthe resultant signal in 4the antenna RA1 will be very low and may, as

experience has shown, be substantially (aero.v y

va pulse transmission receiver.

However. the path B1 toV antenna RAz differs from path A1 by a greateramount-than a Ihalf wavelength and a resultant useful signal willbereceived by RAz. This is apparent from an examination of the geometryof Figure 9 where itis observed thatangle D is larger than angle E. Theheights of the antennas and the reijecting plane C have been greatlyexaggerated with respect to the distance between the transmitter antennaTA and the receiver antennas RA1 and RAe. An Vaccurate geometriccalculation shows that fora transmission distance of 36 miles and afrequency of 4000 megacycles and a separation of 16.5 feet between RA1and vRm that when path B is just. a half wavelength longer than path A,then pathB1 differs from path A1 by an amount greater than a halfwavelength such that the signal over path B1 diifers from the signalover path A1 by a phase angle of 208.5 degrees. This gives a resultantsignal at RAz of 0.5 times the normal value present when multipatheffects vare absent. The equivalent reflecting plane Gmay be at vsuch aheight above the earth that the difference between path BV and path Amay be B72 or 5X; or lf2, etc. of one wavelength which also produces aminimum signal at antenna RA1. It has been found by geometriccalculation that in these cases the received signal at RAa is evengreater than in the case of 1/2 wavelength difference. It is of courseunderstood that the layer C may be such a height as to give a zerosignal at RA2 but in like manner the signal at RA1 isv then of a usablevalue. If the distance between the antennas- RA1 and RAZ is too great,then conditions of zero signal may result at both RA1 and RAzsimultaneously. For example, path length B may differ from A by one-halfwavelength and path length B1 may differ from A1L by 3/2 wavelength. Toovercome this effect it is proposed to restrict the antenna separationto a value less than about 50 feet for rthe example chosen where thedistance is 36 miles and the frequency is- 4000 megacycles.

vAnother method of combining signals which may be used is shown inFigure 6. Here the lower .antenna RA-30I is connected directly inparallel to the higher antenna RA-300. Lower antenna RA30| is arrangedto be mechanically adjusted in position relative to antenna RA-300 in ahorizontal direction so that the phase of the signals picked up by thetwo antennas under normal conditions are additive. This phase adjustmentmay also be accomplished by a variable length in the transmission lineto one of the'antennas or by means of an electrical network connectingthe two antennas together. Then, under conditions which cause a signalresultant of zero on the antenna RA-300, the antenna 11A-30| still sup,-plies a useful .resultant to signal to converter 302. The remainderofthe system shown in Figure 6- may be the same as that describedwithreference to Figure 4 and described in detail in the above-mentionedprior filed Thompson application. It may also be any type of receivingAequipment-such as television with either amplitude or frequencymodulation. It may also be The receiving equipment is generally identiedby reference numeral 309. The receiver may include signal strengthindicating means. It has heretofore been found that the system of Figure6 is not useful at medium frequencies where long distance' circuitsusing diversity antennas spaced in a horizontalplane are used. However,at microwave frequencies and fordistances of the order 'bf 50 miles, the-fconditions 'are l'di'iierent. y .Here "the 4nath length fdiiierencevmay Vmost commonly "beone half wavelength 4orthreeflfialfwavelengthsl'instead of "the di'ierenc'e fof l:man-y wavelengths fwliich Ymay occurlin medium 'frequency llon'g distance circuits. Thus, the destructivecombina- Ation V'of energy from antennas Regino :and -RA-"'u'l l cannottake place.

'The di-versity system shown fin 'Figure 'I A'and `which is usable withfthe-vertically `spaced yari-- torina arrangement of "Figure 2 utilizes,as fbe 'for'e, a pair fof ydirective receiving antennas l#RiLiSllil randRil-3M. ilso'as'before, the signals {pickedup by l"the two antennas farelconverted lto an 'intermediate @frequency rin converters in P302 1 and`362 fbybeating 'them 'with 'locally generated Wave .generated linoscillators 3304 varidi-301i". Alt intermediate frequency the signalsare 'ampli- Tied in fampliers 'BUE and i306 and detected in fluency'modulation carried Thy the signals. VYThe audio `frequency modulationsfare combined "in fa single-output Ichannel v21,!2. -The detectors 408and 1408 in addition Vto separating the foar- A'rier wave from'th-evrn'odiil-ations carried :thereby 'develop adire'c't potential negativelWith respect Ito ground proportional in strength 'to `the amplitude oithe received signals. This negative potential is applied throughvdecoupling network iii-Iii fto fecntro'l thefgain o'fintermediate-amplifiers y Sllandiilth "Then, whensonly-oneoifthe-channels `isfcarrying any .appreciable signals the re suitantdirect V#current r-lciases the `other intermefdia'te frequency ampliiierto such apo'int'thatfit .substantialiy ceases to :amplifyftherebyfcutting 'out-noiseiinfthe idle channel.

fA JVfurther modification shown -in Figure 28 "utilizes the 'sameantenna arrangement and conlvert-ers ras -previously'described- -Here,however, the intermediate lfrequency fampliers 3308 and 30G 'each"contain'limiters. nThe iimiters i develop tnegative potentials inVproportion jto fthe amplitude of the signals-appearing 'fintheampliiers. The 'potential developed in ampliiier "30'6 is rappliedthrough diode i109 to -thegatecontrol or flip-flop 4'circuit i410.v'Similarly the negative jpoftentia'l `developed in vamplifier 7306 isapplied lthrough diode'l'ito thegatefcontrol-t. 'The gate controlAdii-'ll A'selectively operates -g-ates i4 |1| fandA-'I i" @so'thatfon'ly one -o'f the' twoie'hannels "is operative atany-l time -toypro-vide output signals .in-,the foiitput `:lead :5412. The Idiodes#IUS YanoLtlHl ib'eweenfthe 4limiters and gate control 4f-I fprevent theilimiter inamplier e 4from -influeneingthe aoperation of 'Ith'e limiterin amplier 3196' :and Nice-versa. *A lpair foi resistors MSR fand 309B'connected between VVground iand fdio'desf'lUB fand 'M19' respectively:ser-ve as 'load elements forvithe .diodes and form :a `'control voltagey deriving foiricuit. The vpotentials rdevelopedfatthe `ungrouniledvends.of resistors WSR and 4091i @will be-fo'f iii-ke 'negative ipclaritywith lrespect ito ground. i

But .if ...the voltages fare unequal-one l"end of Ithe potential:combining fcircuit rconstituted by the `series Aconnectionfof`resistors :4D BR ran'dllg will c Lbe Tpositive .or :negative :with'respect lto lthe other zend. llipfiicp circuitlltrespondsltofthefchanges 1in relative `polarity of the series connectedV`refsistors MUSE and iliR -c-:onstituting ythe z'control Woltagedesignating :circuit *to open A and flose gates 41 Lanidll I'alternately. Examples-offenen circuitry Will-be foundf'onvreieringto U.SJPatrents 22,106,342, issued January 25, "11938, ito --Stephan Deba,iJr., and "2,51` 5 ,668, issued ily '-18,

Adesired, :a transmitting arrangement .es -showniril'ig-urcelli'mayberused. Herethe transmitter "HM supplies la :pair of antennasTA1 :and TAa'arranged one above the other jas described forthe1receiv'rng antennas Lof lliiguref. Onefo'f the antemiasis arrangedto lbe :shitted along :its line-'of fdirectivity or -a .phase rshiftermay Lhe inserted lvinto one Aof the transmission .lines =feed- `V`ingthe antennas. :The receiverutilizes a-fsi-ngle antenna RAe-3D0feedingfc'onverter 362. VThe =re mainder of the igure corresponds toFigure 6 so :it will 1netbe 'again described. Normallysignals arriveatfRA-il frompnly one Lof the transmit- `ting #antennas "TA1 and1AIA21'lcut1under.abnormal v `propagatioi'i 'conditions when-'destructive `iinteriference takes yplace over this path, the 'otherltransrriitting antenna so Vlocated that :its Ien- -iergy now arrives 'a'IRA-30u andrsupplies ariusa'ble signal ffor ithe duration of ftheinterference.

rThe above-mentioned -fdiversity arrangements Imay also be fused fever-sea, las Well as :over #land where lthe 'communication path may beJbetween two ships at sea lorloetween a land-:stationsandsa ship. Themovement of either 'the transmitter Yor receiver or both, causes thewavefrefiec'ted 'ifromthe surfaceo-f the sea, at'some periodsof time,'tor-'cancel the y'direct Wave. `By zutili-Zing the present-arrangementlth'e eiiects :of such =cancel lation vareenfoided.

"'Phe apparatus-toibo employed 'systems such as'shown-hereinl in`Fi1gure--3 preterablyf'followsthe design =of 3similar apparatus showniin lthe .copending application fof -Leland iE. Thompson,Serial'-No."5-'76g453, ledlFebruary 16, 1'9'45. '.Thus, #Figure 18 may'representithe receiving endfof the relaying system of Figure f2 :of the`-copending Thompson-Yapplication lor 'it may yequally well =.rep resent4the similar lportion :of the receiving -terminal-'of Figure 3 of the`Thompson application. 'Receiving antennas 'fRA- fand TeA-#35| mayfollow ltheFconstruction#shown lin'lFigure "1'0.of `the 'Thompsonapplication. The Wavesfreceived by the receiving antennas BLA-13W V:andRik-L3M Iof rFigure "3 of this vapplica.'tionfnray :have .the :carrieryfrequencies and ddeviations yorY-Iri'odulations such las -d'es'criloedin vthe Thompsonapp'lication. The `localoscillators 3045304 `and.362,1.'aiiimay follow vthe 'construction 'and operation fof-theapparatus diagrammaticaliy illustrated iin Figure "f7 "of `the `Thompsonapplication. The intermediate ire- -iquency amplier 30G, 306' may be:designed lto `#follow the constructionlof the intermediate tre- Aqueneyamplifier "206 Cof @FigureZ o'f-the l'fhompson application or amplifierf`3'06 of 'Figure i3 of the Thompsonapplication.

'The A discriminator detector 'apparatus F308; 30 8 Yo'f yFiguredof'thisapplication mayfollowthe-fdesign-and constructionof#similarapparatusfshown VinfFiguie E8B yof l the `copending Thompsonl'application.

lHo-weven-it 'isto be -clearly understoodfthat'the *present Lsystem forreducing Ithe 'eiectsfo'f fading when signalling with 'Very vshort Wavesis not limited to la 'multiplex lsystem or to v-a rmultiple fmodulationsystem euch as adescribed 2in .=the Thompson application although-particulariyiusev`itil therefor., -fbutinray be used equally-'wellfforother systems such as femployedffcr rsimplex signailing vmaking use FofSingle modulation `or fdirect 1 modulation of the emitted -or"transmitted carrier'tvaveioyfthe signal. "Similarly, it -is to'beunderstoodlthatfthefapparatusitself'needinotol- W that o'f the" Thompsonapplication, Vbut other forms f-'offcons'truction of the apparatus smayhe used,

While we have illustrated'several embodiments ,of` the vpresent.invention, it ,should be clearly understood that it is not limitedthereto since many modifications may be made in the several elementsemployed and in their arrangement without departing from the spirit andscope of the invention. Y -Y frWhat is claimed is: l f 'f 1. An ultrahigh frequency communication system including, at one location, atransmitter of double angle modulated wave energy and, at anotherlocation, a receiver for said energy,I an antenna'at said receivinglocation arranged to receive substantially all of said' wave energyVVover one path between the transmitter at said one location and saidantenna at said receiving location under conditions of normal wavevpropagation between said locations, a second antenna arranged at saidreceiving location to receive substantially all of said wave energy overa second under abnormal wavel propagation conditions,

said paths having different lengths to effect destructive interferenceYat said receiving location,V

said antennasbeing spaced in the vertical direction, means separately toconvert the individual signals received by said-twol antennas toL a cornmon intermediate frequency,- intermediatev` frequency ampliers includinglimiters to amplify said converted signals, means to apply saidamplified signals to a common output channel, an electronic gate circuitinterposed between each of said amplifiers and said common channel,rectifier elements individually coupled to said amplifiers, loadresistors coupled between said rectifiers and a point of fixed referencepotential and a iiip flop circuit coupled to the junction points betweensaid resistors and said rectiiiers and connected to said gate circuitsto open and close the same alternately in response to the potentialsdeveloped across said resistors.

2. An ultra high frequency communication system including, at onelocation, a transmitter of wave energy and, at another location, areceiver for said energy, an antenna at said receiving location arrangedto receive substantially all of said wavek energy over one path betweenthe transmitter at said one location and said antenna at said receivinglocation under conditions of norf mal wave propagation between saidlocations, a

second antenna arranged at said receiving location to yreceivesubstantially all of said wave en-` ergy over a second path between thetransmitter at said one location and the other antenna at said receivinglocation under abnormal wave propagation conditions, said paths havingdifferent l0 saidv load elements and the output vcircuit thereof coupledto said electronic gate circuits selectively to open and close the samein response to the resultant of said voltages developed across said loadelements to apply the signal of higher level only to said commonlchannel.

3. In an ultra high frequency diversity receiving 'system includingmeans to receive individual signals of given" desired wave energy underconditions normally producing destructiveA interference, meansseparately to convert the in# dividual signals Areceived to a commoninter-'- mediate frequency, intermediate frequency amplifiers includinglimiters to amplify said-coni verted signals individually, and means toapply said amplified signals to a common output chan'- nel,a`nelectronic gate circuit interposed between each of said intermediatefrequency amplifiers and said commonl output channel, a rectifiercoupled to eachv of said limiters, a control voltage discriminatingcircuit coupled to said rectifiers and a flip-flop circuit coupled tosaid gate cir= cuits and said discriminating circuit effective to applyonly theampliiied signal'having the greater amplitude to said commonoutput channel;

4. In a wavelength modulation diversity '1re'- ceiving system havingmeans producing two sig'- nal voltages representative of the same signalbut varying in qualitative sense, means individually limiting theamplitude of said voltages, and an output channel, means to select thebetter of the two voltages including an electronic gate coupled betweeneach of said limiting means and said output channel, an electronicflip-flop circuit coupled to said electronic gates, a rectifier circuitcoupled to each of said limiting means t0 produce a potentialproportional to the amplitude of the respective signal voltage at theoutput of said limiting means, a potential combining circuitinterconnecting said rectifier and said flip-flop circuit to apply acontrol potential thereto to actuate said electronic gates to applythesignal voltage of greater amplitude only to said output channel.

5. In an ultra-high frequency diversity receiving system including meansto receive individual signals of given desired wave energy underconditions normally producing destructive interference, means separatelyto translate the signals received, amplifiers including `limitersindividual to said signals and coupled to said translating means, gatingmeans coupled to said limiters to apply said translated signals to acommon output channel, a rectier coupled to each of said translators,resistors connected in series between said rectifiers with the junctionbetween the resistors connected to a point of fixed reference potential,and a flip-nop circuit coupled to said gating means and across saidseries connected resistors effective to apply only the translatedsignalhav- 1 ing the greater amplitude to said common output limiters toamplify said converted signals, means l I Y across individuallyproportional to the amplitude of the signals in said' amplifiers, and agate con trol circuit arrangement having the input circuitv thereofconnectedacross the series connection of channel.

6. In anv ultra-high frequency diversity receivditions normallyproducing destructive interference, means to convert separately thesignals received to a common intermediate frequency, intermediatefrequency ampliers including limiters to amplify said converted signalsindividually, and electric path connections between amplifiers and acommon output channel, an` electronic gate circuit interposed betweeneach of said amplifiers and said common output channel,

Il asrectiercoupleditaeach otsactampliersgaef sistorcormectedl between.a: point of.. xepd. poten:- tixaflfandeaclr.. of; said.' rectiers andaflip opcircoupleditcr saidf. gate. circuits: and: across;saidsetiesfconnectedtresistorseiectivetprevent only the amplified signalhavirrgr thev lesser: amplitude frnmfreaching: said? common output.'`channel...

'ILiIr an ultra-,high frequency' communication system includinemeansftos receive individuaL sig of given: desired; Wave energy; underVcondi.- titans-f3ircrirrallitproducing, destructiveinterferermampiiflers includinglimitersatofamplify/said receivedisignalsindividually; anclmeans to. apply sainz amplid" signals tor alcommon`output chan.-

arr. electronic: gatei circuit'. interposed between.

each; ot said intermediateamplifiers.. and: said commun; autputichannel,ai rectifier coupled. to eaehofsaid. ampliflers'a.controtvoltagederiving circuiti conpl'erlztesaiclzrectiers. saidA circuit. com.-prrisi-ngfV resistors' connected; betweenf saidrectiers andcpoint offxedreference potential; the; control voltagefbeingderived-z across theseriesconnection of saidi `nesis'tors,l andv a, flip-Hop circuit`coupled tarsairt gate; circuits and saidv control voltage? deriving;circuit. effect-ive toA applyonly thev ampli.- fied signal: having. thegreater: amplitude to said common output channel.

DONALD S; BOND.'` LELAND E.. THOIVIPSON.

CITED.-

Tliefollowing' references` are: of" record the leof thisl patent:

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